The present invention relates to a time division multiple access (TDMA) radio communication system and, more particularly, to an improvement in channel assignment, i.e., slot assignment of the TDMA radio communication system.
As is well known, in a TDMA radio communication system, a plurality of stations connected through the TDMA channel are constituted by one (or two or more) master station and a large number of slave stations, each of which receives a reference signal transmitted from the master station and sets a transmitting/receiving timing of its own. Each slave station transmits a communication signal in a burst format in a channel, i.e., a time slot, which is time-divisionally assigned to the slave station, thus performing communication. A demand assignment system for assigning a time slot in response to each assignment request from the slave station is known as a system of assigning a slot to each slave station. This system is well known in a satellite communication system.
FIG. 1 shows a transmitting operation sequence of a TDMA radio communication system employing a conventional demand assignment system. In FIG. 1, when a calling station receives a transmitting signal from a terminal or an exchanger and detects a calling signal therefrom in step (A), it sends to a master station a channel connection request added with an identification number of its own and an identification number of a slave station serving as a called station. In step (B), the master station performs channel connection control for checking availability of the channel, i.e., searching of the presence/absence of vacant time slots and a connection condition of the channel, i.e., checking whether or not the receiving station is busy. If a vacant slot is found and the receiving station is not busy, the master station sends back an acknowledge response (ACK) for the channel connection request to the calling station. In step (C), the calling station sends to the master station a slot assignment request prepared by adding a necessary time slot count to an identification number of the slave station serving as the called station.
In steps (D) and (E), the master station performs channel control. That is, the master station retrieves a channel assignment condition, determines a slot to be assigned, and sends slot assignment information to the calling and called stations. The slot assignment information sent to the called station includes assignment information of a slot used when the called station transmits data, and information indicating a slot assigned to the calling station.
Similarly, slot assignment information sent to the calling station includes assignment information of a slot assigned to the calling station and assignment information of a slot assigned to the called station. More specifically, the calling and called stations send a burst signal to a predetermined time slot, and fetch it therefrom, thereby the calling and called stations are associated with each other using the time slot. In response to the slot assignment information, the calling station sends the ACK to the master station in step (F), and the called station sends the ACK to the master and calling stations in step (G). In step (H), the calling station sends data transmission start information to the called station at a given timing. In step (I), the called station sends to the calling station the ACK indicating that reception is ready.
In step (J), the calling station sends transmission signals DATA1 to DATAn to the called station using the assigned time slot. In step (K), since transmission is completed, the calling station performs channel disconnection control for sending a disconnection request to the master and called stations. In step (L), the called station similarly performs channel disconnection control for sending a disconnection request to the master and calling stations. In step (M), the master station receives the disconnection requests from the calling and called stations, and releases the assignment of the time slot assigned to these stations.
To summarize, in the demand assignment system, the master station performs channel assignment control for a plurality of slave stations. In the conventional demand assignment system, a reception enable/disable state of the called station is checked during channel connection control in addition to availability of the channel. During channel assignment control, a channel assignment condition is grasped and retrieved to assign a slot to the calling station, and slot assignment is performed for the called station, so that correlation between the calling and called stations is established with reference to the time slot. During transmission/reception end processing, the disconnection requests from both the calling and called stations are received to release slot assignment.
However, in the TDMA radio communication system employing the conventional demand assignment system, the following problems are posed.
Since the master station controls availability of the channel, i.e., availability of time slots and a connection condition between the calling and called stations, it must process a large amount of data. When a plurality of stations issue channel connection requests and channel assignment requests at the same time, the processing requires much time, and a queue is easily formed. As a result, it is difficult to quickly perform channel setting and channel assignment.
In order to avoid this, the master station must have a high-grade function such as a parallel processing or high-speed processing function, resulting in an expensive master station.
Since a connection condition between the calling and called stations is controlled, the channel connection request of a new calling station is not accepted unless the receiving station is ready for reception. Even if the channel connection request or channel assignment request is accepted, signal transmission is performed after a response from the called station is received. Therefore, it takes much time from generation of calling until signal transmission, resulting in a long response time. As a result, the queue is more easily formed. In particular, in a satellite communication system, since earth stations are discretely located, the calling and called stations may have a difference in weather. Under this condition, taking into consideration the fact that the master or calling station may fail to receive a response from the called station, the above problem is very important.
Since the calling and called stations are correlated with reference to a time slot, when the calling station changes a called station, the channel assignment procedure and the like must be retarded. For this reason, channel control becomes complicated, and it is difficult to improve transmission efficiency. In other words, a called station cannot be changed during a series of signal transmission procedures.